Shadow transits of Moons of Jupiter on Jupiter are relatively common.
Double shadow transits are somewhat less common.
Seen here is the moon Io off to the right with two shadows, one from Callisto and the other from Io, also off to the right. The Great Red Spot appears only in the first few image runs, then goes around the edge.

The double transits and the Great Red Spot were to begin at 17:25 EDT but it was still daylight. The telescope pointing could not find Jupiter, not until 18:27 when Jupiter was visible to the naked eye and the Telrad finder helped aim the scope.
Images were taken every five minutes until the end of the event at 19:25 EDT. So instead of 2 hours, only 1 hour of images were captured.

Images runs were 180 seconds each and processed with PIPP, Autostakkert! and Registax for wavelets.
An animated .gif of the entire event is below: